The present disclosure relates to structures each of which includes a gallium nitride-based semiconductor layer having a surface that is a nonpolar plane or a semi-polar plane, and includes a metal layer provided on the surface of the gallium nitride-based semiconductor layer.
Nitride semiconductors containing nitrogen (N) as a group V element have been expected as a material of a short wavelength light-emitting element because of their band gap size. In particular, gallium nitride-based compound semiconductors (GaN-based semiconductors) containing Ga as a group III element have been intensively studied, and blue light-emitting diode (LED) elements, green LED elements, and semiconductor laser elements formed of GaN-based semiconductors have also been commercialized.
GaN-based semiconductors have a wurtzite crystal structure. FIG. 1 schematically illustrates a unit lattice of GaN. In AlxGayInzN semiconductor crystal (where 0≦x<1, 0<y≦1, 0≦z<1, and x+y+z=1), some of Ga atoms illustrated in FIG. 1 may be substituted with at least one of Al or In.
FIG. 2 shows fundamental vectors a1, a2, a3 and c of the wurtzite crystal structure. The fundamental vector c extends in a [0001] direction, and this direction is referred to as a “c-axis.” A plane perpendicular to the c-axis is referred to as a “c-plane” or a “(0001) plane.” A plane terminated with group III elements, such as Ga, is referred to as a “+c-plane” or a “(0001) plane,” and a plane terminated with group V elements, such as nitrogen, is referred to as a “−c-plane” or a “(000-1) plane,” and these planes are distinguished from each other.
In the case where a semiconductor element is fabricated using a GaN-based semiconductor, in general, a c-plane substrate, that is, a substrate having a (0001) plane as a growth surface is used as a substrate on which a GaN-based semiconductor crystal is grown. However, in the c-plane, Ga atoms and nitrogen atoms are not present on the same atomic plane, and therefore, electrical polarization occurs. For this reason, the “c-plane” is also referred to as a “polar plane.” As a result of the electrical polarization, a piezoelectric field is generated along the c-axis in an InGaN quantum well layer included in an active layer of the gallium nitride-based semiconductor light-emitting device. Due to the piezoelectric field generated in the active layer, electrons and holes distributed in the active layer are displaced, and the internal quantum efficiency of the active layer is decreased due to a quantum-confined Stark effect of carriers. This increases a threshold current in the case of a semiconductor laser element. This also increases power consumption and reduces luminous efficiency in the case of an LED. This further increases implanted carrier concentration, piezoelectric field screening, and a change in light emission wavelength.
To solve these problems, using a substrate (an m-plane GaN-based substrate) having a nonpolar plane as its growth surface, e.g., a (10-10) plane called an m-plane that is perpendicular to the [10-10] direction, has been considered. The sign “-” given to the left side of an index of Miller indices in parentheses indicates a “bar (inversion)” of that index, and corresponds to the “bar” in the drawing. As illustrated in FIG. 2, m-plane is in parallel with c-axis and is orthogonal to c-plane. In the m-plane, Ga atoms and nitrogen atoms are present on the same atomic plane, and therefore, spontaneous electrical polarization does not occur in a direction perpendicular to the m-plane. This means that a piezoelectric field is not generated in the active layer, and the above problems are solved, if a stacking semiconductor structure is formed in the direction perpendicular to the m-plane. The m-plane is a collective term for (10-10) plane, (−1010) plane, (1-100) plane, (−1100) plane, (01-10) plane, and (0-110) plane.
As illustrated in FIG. 3C, a-plane is in parallel with the c-axis (a fundamental vector c) and is orthogonal to the c-plane illustrated in FIG. 3A. The a-plane is a collective term for (11-20) plane, (−1-120) plane, (1-210) plane, (−12-10) plane, (−2110) plane, and (2-1-10) plane.
FIG. 3D illustrates r-plane, which is a collective term for (10-12) plane, (−1012) plane, (1-102) plane, (−1102) plane, (01-12) plane, and (0-112) plane.
Further, −r-plane is a collective term for (10-1-2) plane, (−101-2) plane, (1-10-2) plane, (−110-2) plane, (01-1-2) plane, and (0-11-2) plane.
Japanese Unexamined Patent Publication No. 2005-197687 discloses a technique that uses a structure of an antioxidation electrode/an aggregation-prevention electrode/a reflecting electrode/a contact electrode/a p-type GaN, to prevent aggregation of the reflecting electrode formed of silver (Ag), rhodium (Rh), aluminum (Al) or tin (Sn).
Jun Ho Son, Yang Hee Song, Hak Ki Yu, and Jong-Lam Lee, “Applied Physics Letters” Vol. 95, P. 062108, Aug. 14, 2009 discloses a technique in which in order to increase a reflection coefficient of an Ag electrode and reduce contact resistance, a nickel (Ni) layer is formed on an interface between a GaN layer and an Ag layer, thereby promoting crystallization of Ag.
Japanese Unexamined Patent Publication No. 2010-56423 discloses an electrode for a semiconductor light-emitting device, and the electrode includes an Ag alloy layer added with palladium (Pd) and copper (Cu) or germanium (Ge), using Ag as a principal component, to achieve both of a high reflection coefficient and a low contact resistance of the electrode.
Japanese Unexamined Patent Publication No. 2010-062274 discloses a semiconductor light-emitting diode which includes: a stacking structure including an n-type semiconductor layer, a p-type semiconductor layer, and a light-emitting layer provided between the n-type semiconductor layer and the p-type semiconductor layer; a first electrode connected to the n-type semiconductor layer and containing at least one of silver or a silver alloy; and a second electrode connected to the p-type semiconductor layer.
Japanese Unexamined Patent Publication No. 2012-080142 discloses a method for manufacturing a semiconductor light-emitting diode which includes: a stacking structure including a first semiconductor layer of a first conductivity type, a second semiconductor layer of a second conductivity type, and a light-emitting layer provided between the first semiconductor layer and the second semiconductor layer; and an electrode provided on the second semiconductor layer disposed opposite to the light-emitting layer. In this publication, a first metal layer containing silver or a silver alloy is formed on a surface of the second semiconductor layer disposed opposite to the light-emitting layer, and a second metal layer containing at least one element of platinum, palladium and rhodium is formed on the first metal layer. The second semiconductor layer, the first metal layer, and the second metal layer are sintered in an atmosphere containing oxygen. The sintering temperature is such a temperature that makes an average particle diameter of silver contained in the sintered first metal layer is not more than three times an average particle diameter of the silver before sintering.
Japanese Unexamined Patent Publication No. H10-200161 discloses performing oxygen plasma ashing on a surface of an n-type GaN contact layer, and thereafter forming an electrode in which a Ti layer, an Al layer, a Pt layer, and an Au layer are sequentially formed.